1. Technical Field
This invention relates generally to reproduction apparatus, such as for example copiers and nonimpact printers of the type which forms a plurality of images on an endless image member and then transfers those images in registration to a transfer surface. The invention is particularly usable in color reproduction.
2. Background Art
U.S. Pat. No. 4,712,906 issued Dec. 15, 1987 to R. Borruso and J. Butler shows a color printer having an electrophotographic endless web entrained about a series of rollers. Electrostatic latent images representing color components of a desired multicolor image are formed by uniformly charging the web and exposing it using an LED printhead. Consecutive latent images (or pairs of images) are toned with different colored toners, and the toned images are transferred in registration to a transfer surface brought into contact repeatedly with the images by a transfer drum. The single-color images are referred to herein as "color separation" images.
Registration of the color separation images is achieved in part by an encoder which triggers exposure of the web according to the angular position of a roller associated with the printhead. The printhead roller is connected by gearing or a timing belt to the transfer drum so that the transfer surface is presented in substantial registration with the beginning of the first image. The accuracy of placement of subsequent images on top of the first image is dependent upon consistency in the movement of the web between the printhead and the transfer station. With this structure, small errors associated with drum diameter differences, errors in the drive train between the transfer drum and the printhead roller, and the like, cause registration errors which accumulate over the course of formation of a set of multicolor images.
U.S. Pat. No. 4,025,186 shows an endless web electrophotographic member having a series of perforations along one edge which perforations are sensed to control actuation of work stations along the web. U.S. Pat. No. 4,477,176 shows a similar approach for controlling superimposed multiple image apparatus, for example, a multicolor image copier.
U.S. Pat. No. 4,821,066 discloses a nonimpact color printer having a web upon which two or more images are formed in response to an electronic signal and transferred in registry. Actuation of the printhead is controlled in response to sensing the leading or trailing edge of each perforation as it passes a predetermined position associated with the print head and the transfer surface.
In commonly assigned U.S. patent application Ser. No. 07/239,885, flash exposure of each frame is delayed by an independently adjustable duration after sensing an indicia. An operator or service representative installing a new web runs color reproductions using an appropriate color original, examines the reproductions for color registration and makes the appropriate adjustments in the delay time to give high quality. As good as this apparatus is, it is only as accurate as the resolution powers of the error detection means; be it the operator or an apparatus. While these errors are relatively small and not unacceptable for much color work, they prevent forming images of highest quality with ordinary error detection methods.